Coolant circuit for resistive cryogenic electric power transmission line

ABSTRACT

A coolant circuit for refrigerated electric power transmission cable utilizing coolant in the cable in which capacity to accommodate cyclic peak loads, overloads, and emergency conditions is provided; 1. BY OPERATING THE CABLE COOLANT CIRCUIT WITH A LIQUID-PHASE COOLANT IN THE CABLE UNDER SUPERATMOSPHERIC PRESSURE AND NORMALLY MAINTAINING THE TEMPERATURE OF THE COOLANT SUBSTANTIALLY BELOW ITS VAPORIZATION TEMPERATURE SUCH THAT CYCLIC PEAK LOADS AND TEMPORARY OVERLOADS AND EMERGENCIES CAN BE ACCOMMODATED BY THE THERMAL MASS OF THE COOLANT, I.E. BY ALLOWING THE TEMPERATURE OF THE COOLANT TO RISE WHILE MAINTAINING IT IN THE LIQUID PHASE, 2. BY FLASHING REFRIGERANT CONTAINED IN A RESERVOIR LOCATED EXTERIORLY OF THE CABLE TO OBTAIN ADDED COOLING CAPACITY IN CASES OF PROLONGED OVERLOAD AND EMERGENCY, AND 3. BY EMPLOYING PARALLEL ELECTRICAL CIRCUITS IN A COMMON COOLANT LOOP SUCH THAT THE ENTIRE REFRIGERATING CAPACITY CAN BE UTILIZED TO COOL ONE ELECTRICAL CIRCUIT IN THE EVENT OF BREAKDOWN OF THE OTHER. Accommodation to long term increase in required cable capacity is also provided.

United States Patent Graneau et al.

[54] COOLANT CIRCUIT FOR nizsrsrrvr: CRYOGENIC ELECTRIC POWERTRANSMISSION LINE [721 Inventors: Peter Graneau; Joseph L. Smith, Jr.,both of Concord, Mass.

[73] Assignee: Simplex Wire and Cable Company,

Burlington, Mass.

[22] Filed: Oct. 27, 1969 [21] Appl. No.: 869,476

[52] US. Cl. ..174/15, l74/DIG. 6, 62/514 [51] Int. Cl. Jllllb 7/34 [58]Field ofSearch ..174/l5, 15C, 15 SC, 16, 16 B;

[56] References Cited UNITED STATES PATENTS 3,463,869 8/1969 Cooley eta1 ..174/l5 3,461,218 8/1969 Buchhold ..174/l5 3,396,551 8/1968Dimentberg.... ....174/15 X 3,375,675 4/1968 Trepp et a1 ....62/514 x3,292,016 12/1966 Kaflta ...174/l5 X Primary ExaminerLewis H. MyersAssistant Examiner-A. T. Grimley AttorneyMcLean, Morton and Boustead flei i g iei [451 Feb. 29, 1972 [57] ABSTRACT A coolant circuit forrefrigerated electric power transmission cable utilizing coolant in thecable in which capacity to accommodate cyclic peak loads, overloads, andemergency conditions is provided;

exten'orly of the cable to obtain added cooling capacity in cases ofprolonged overload and emergency, and

3. by employing parallel electrical circuits in a common coolant loopsuch that the entire refrigerating capacity can be utilized to cool oneelectrical circuit in the event of breakdown of the other.

Accommodation to long term increase in required cable =Patenfed, Feb.29; 1972 I 3,646,243

2 Sheets-Sheet 1 INVENTORS I PETER GRANEAU JOSEPH L. SMITH,JR.

0% #414 wgmzw ATTORNEYS Patented Feb. 29, 1972 3,646,243

2 Sheets-$heet 2 FIG.3

TO ATMOSPHERE I n'wam'ons PETER GRA NEAU J OSEPH L. SMITH JR.

ATTORNEYS COOLANT CIRCUIT FOR RESISTIVE CRYOGENIC ELECTRIC POWERTRANSMISSION LINE This invention relates to electric power transmissionand has particular applicability to circulation of a coolant in acoolant circuit in heat exchange relationship with conductors of atransmission line.

This invention is specifically directed to the problems in a resistivecryogenic electric power transmission line created by cyclic loadvariations, overloads, and emergencies, and has for its object thedesign of a coolant circuit in which the refrigerating capacity utilizedto extract heat from the coolant is designed for less than the heatgenerated at anticipated cyclic peak load and approximates that of someintermediate anticipated operating load, but which coolant circuit iscapable of accommodating cyclic peak loads of diurnal, weekly,

and seasonal nature as well as overloads and emergency conditions.

In another aspect of this invention, it is an object to provide acoolant circuit for a resistive cryogenic electric power transmissionline in which temporary breakdown in one power circuit can beaccommodated by increasing the capacity of one or more parallel powercircuits.

It is also an object of this invention to provide coolant circuits forresistive cryogenic electric power transmission lines in which the powerhandling capacity of a transmission line can be increased to accommodatelong term increases.

With the increasing cost of the acquisition of land for right of waysrequired for aerial cable installations, attention has been directed tothe employment of underground electric power transmission in suburban,as well as urban areas. The manufacturing costs of paper-insulated cableand the size of the conductor required to achieve a given load capacitywhen the only heat sink is the ambient soil have also lead toconsideration of the use of underground cables employing coolant systemsin which Joule heat losses in the conductor are extracted by a flowingcoolant, e.g., a liquid cryogen. One particularly suitable cable forunderground use utilizing a circulating coolant is the high-vacuuminsulated cable in which the conductors are tubular and are supported ina vacuum spaced one from the other, with suitable shielding whererequired, within a buried pipe.

This invention has particular applicability to such highvacuum insulatedcable, whether used in an underground installation or not, but is alsogenerally applicable to cables in which a conductor is cooled bycirculation of a coolant, particularly a cryogen in the liquid phase, inheat exchange relationship with one or more conductors. The invention insome aspects, however, is also applicable to use of gas-phase coolantsand two-phase coolants, i.e,, liquid gas mixture and liquid-solidmixtures where part or all of the heat is absorbed by change of phase ofthe coolant.

With these generalities in mind, further discussion below will beusually confined to description of AC transmission lines involvingthree-phase circuits in which each conductor is tubular in shape, isconstructed of high-purity aluminum, e.g., 99.9 percent pure anodizedaluminum, and is cooled by circulation through its center of liquidnitrogen maintained under superatmospheric pressures and at temperaturesbetween 63 K. and 105 K. This invention, however, has equalapplicability to other conductor materials with which other coolants arecirculated in heat exchange-relationship.

In accordance with this invention, there is provided both a coolantcircuit for a resistive cryogenic electric power transmission line and amethod of operation of such circuit to accommodate cyclic peak loads,overloads, and emergency conditions. It will be understood by resistivecryogenic electric power transmission line reference is intended toelectric cables operated to carry large quantities of power in whichheat is extracted from the current-carrying conductors by circulation ofa refrigerated fluid, i.e., coolant in heat exchange relationship withthe current-carrying conductors such that the conductors are operated attemperatures substantially below those of the environment but usuallyabove those in which the conductors exhibit superconducting properties.In certain aspects, however, it will be apparent that operation attemperatures in the superconducting range can be employed.

In accordance with one aspect of this invention, it is contemplated thata coolant circuit will be employed to circulate a coolant, which ismaintained at a superatmospheric pressure to hold it in the liquid phasein heat exchange relationship with the conductor to be cooled.Refrigerating capacity is provided, for example, in the form of arefrigerating plant including a cryogenerator, Le, a refrigerator forextracting heat at low temperatures, such that, under normal loadconditions, the refrigeration capacity is adequate to maintain thetemperature of the coolant circulating in heat exchange relationshipwith the conductor at a temperature substantially below the vaporizationtemperature of the coolant at the lowest pressure encountered while inheat exchange relationship with the conductor. The mass of coolantemployed is selected taking into consideration the cooling capacityrequired in excess of the refrigerating capacity of the installation atcyclic peak loads anticipated and taking into consideration the durationof such cyclic peak loads in excess of refrigerating capacity such thatthe excess cooling capacity required over that supplied by therefrigerating capacity is accommodated by allowing the temperature ofthe coolant to increase to some temperature less than the maximum atwhich the coolant remains in the liquid state under the pressureconditions to which it is subjected while in heat exchange relationshipwith the conductor.

In other words, refrigerating capacity of the refrigerating plant isselected between that cooling capacity required to carry the cyclicminimum loads and that required to carry the cyclic peak loads suchthat, during operations in which cooling capacity required is less thanrefrigerating capacity of the refrigerating plant, the coolant islowered in temperature, in effect, to store cold" which then can beutilized to supply the added cooling capacity required in excess of therefrigerating capacity of the plant during periods of peak loads. Thesame stored cold" is available to meet temporary overloads and otheremergencies.

In another aspect of this invention, it is contemplated that substantialquantities of additional coolant will be stored exteriorly of the cablein suitable reservoirs typically located at the site of therefrigerating plant or plants such that the refrigerating capacity ofthe plant can also be utilized to refrigerate the reservoir coolant andstore additional cold in excess of that required normally to maintainoperation of the cable in accommodating normal cyclic load variations.In this aspect of the invention, when unusual peak loads are encounteredor prolonged overloads are present because of emergency conditions, theadditional coolant in the reservoir is a liquefied refrigerant such thatthe stored cold in the reservoir of liquid refrigerant can also beutilized, even when its temperature approaches that of vaporization byflashing the refrigerant in the reservoir to reduce the temperature ofthe remaining liquid in the reservoir through the release of the heat ofvaporization of the flashed refrigerant. In this aspect of theinvention, the refrigerant in the reservoir can be the same material asthe coolant employed in the coolant circuit, but the coolant circuit canalso use different coolants, such as gasphase substances and the like.

In yet another aspect of this invention, it is contemplated that eachresistive cryogenic electric power transmission line will include atleast a pair of electrically parallel electrical power circuits of twoor more conductors such that coolant is circulated in a loop in onedirection through the conductors of one power circuit and in the reversedirection through the conductors of another power circuit with therefrigerating plant located at one end of or both ends of the powercircuits, or at some intermediate position along the power circuits. Insuch an arrangement, if there is a failure in one power circuit causingits breakdown, all the coolant can be circulated in both directions inthe conductors of the other circuit thereby increasing the capacity ofthat circuit to absorb Joule heating losses such that it can accommodatethe load of both circuits,

at least for limited periods of time, to enable repair of the circuitwhich has broken down.

In a further aspect of this invention, when all other means have failed,added refrigerating capacity can be provided by filling additional coldliquid refrigerant into the reservoir without disturbing operation ofthe transmission line. Liquid nitrogen is generally available, and thisavailability is one of the advantages of the material as a coolant inthe systems of the present invention. I

In another aspect of this invention, it is contemplated that long termgrowth in electrical loads which must be handled by most giventransmission lines can be accommodated simply by adding additionalrefrigerating capacity in the form of additional refrigerating plants atthe location of the original refrigerating plant or at new locationsspaced along the length of the transmission lines at closer intervalsthan the original design.

For a more complete understanding of the practical application of thisinvention, reference is made to the appended drawings in which:

FIG. 1 is a schematic diagram of a resistive cryogenic electric powertransmission line in accordance with this invention;

FIG. 2 is a schematic diagram of a portion of the apparatus shown inFIG. 1;

FIG. 3 is a cross section of a three-phase circuit, such as is shown inFIG. 1;

FIG. 4 is a schematic diagram of a cryogenerator suitable for use inconnection with the apparatus shown in FIG. 2; and

FIG. 5 is a modification of the transmission line shown in Fig. 1 toprovide added capacity in accordance with the invention.

Referring to FIG. 1, a coolant circuit in accordance with the presentinvention includes two electrically parallel, threephase power circuitsl0 and 11 which are physically disposed side by side buried in a commontrench. The actual electrical terminations of power circuits l0 and 11are irrelevant to the present invention except that, in accordance withone aspect of this invention, they are arranged each to carry one-halfof the power load of the transmission line.

Referring more particularly to FIGS. 2 and 3, each power circuit and 11(power circuit 10 only'being shown in FIG. 3, although it will beunderstood power circuit 10 is physically identical with power circuit11) includes three tubular electric conductors 12 which are disposedwithin buried steel pipe 13. Each conductor 12 is 99.9 percent anodizedaluminum and is supported at intervals along pipe 13 by means of spacers14 such that conductors 12 are disposed equidistant one from the otherand each from the walls of pipe 13 and are also electrically isolatedone from the other and from pipe 13. Spacers 14 preferably are of thetype disclosed in copending Graneau application Ser. No. 727,993, filedMay 9, 1968, now U.S. Pat. No. 3,542,938. lnteriorly, pipe 13 is linedwith analuminum eddy current shield 15 in the form of an aluminum tubeand an interposed layer of thermal insulation 16. The interior of eachpipe 13 and shield 15 is operated at high vacuum on the order of IO to10' millimeters of mercury.

The coolant circuit of this invention further includes a refrigeratingplant 17 for supplying liquefied nitrogen, LN at superatmosphericpressure through a line 18 and a manifold 19 to the ends of conductors12 at one pair of adjacent ends of power circuits l0 and 11.Refrigerating plant 17 is further designed to withdraw liquid nitrogen,LN from the ends of conductors 12 in power circuit 11 at the sameadjacent pair of ends of circuits l0 and 11 through a manifold 20 and aline 21. Refrigerating plant 17 is arranged, as described below, to coolthe liquid nitrogen, LN withdrawn through line 21, to repressure it, andto return it to line 18. Connections are provided, as indicated byreference numeral 22, to circulate liquid nitrogen from conductors 12 ofpower circuit 10 to conductors 12 of power circuit 11 at the adjacentpair of ends of circuits 10 and 11 remote from refrigerating plant 17.

Referring more particularly to FIG. 2, refrigerating plant 17 includes acirculating pump 24, a liquid nitrogen reservoir tank 25, acryogenerator 26, and a heat exchanger 27. Pump 24 is arranged with itssuction side connected to line 21 and its discharge side connected to aline 28 leading to a three-way regulating valve 29 arranged in one modeto connect line 28 to a line 30 leading through heat exchanger 27 toline 18. In its other mode, valve 29 connects line 28 to a line 31leading through a coil 32 disposed in reservoir 25 and thence back toline 30, heat exchanger 27 and line 18. Reservoir 25 is adapted toreceive and retain at a low temperature a large supply of liquidnitrogen, LN and contains a coil 33 as well as coil 32, each of whichare positioned to be in heat exchange relationship with the supply ofliquid nitrogen retained in tank 25. In addition, tank 25, which isconstructed to withstand internal subatmospheric pressure, is providedwith a vent valve 34 located to communicate the upper interior of tank25 through line 47 with the atmosphere when opened.

Cryogenerator 26, referring to FIG. 4, is operated utilizing neon gas asa refrigerant and includes a heat exchanger 35, having three heatexchange elements 36, 37, and 38, a compressor and aftercooler 39 and aturboexpander 40 which are connected to extract heat from tank 25 andfrom heat exchanger 27. The neon cycle refrigerator requires a closed,hermetically sealed system. Thus the intake side of cryogenerator 26, asindicated by line 44, leads from coil element 33 in tank 25 throughelement 37 of heat exchanger 35 to compressor 39 which discharges vialine 45 through element 38 to expander 40. The last discharges throughline 46 to a three-way regulator valve 56 which, in first mode, leadsthrough line 57 to a three-way regulatory 58. In its first mode, valve58 connects line 57 through coil 33 to line 44 and, in its second mode,valve 58 connects line 57 directly to line 44 and the return tocryogenerator 26. In the second mode of valve I 56, line 46 is connectedthrough heat exchanger 27 to line 57 and then to valve 58. Vent valve 34is connected to atmosphere through line 47 and element 36 of heatexchanger 35.

Connections are provided to permit circulation of liquid nitrogen ineither circuit 10 or circuit 11 to the exclusion of the other by meansof a valve 60 in line 18, a valve 61 in line 21, a valve 62 in oneconductor 12 of circuit 10 located adjacent manifold 19, a valve 63 inone conductor 12 of circuit 11 adjacent manifold 20 and a valve 64 inline 22. Each of valves 60, 61, 62, 63 and 64 is normally in a positionpassing flow in the line or conductor in which it is connected. Valve 64has a second position closing flow in line 22. A line 65 interconnectsnormally closed ports in valves 60 and 63, and a line 66 interconnectsnormally closed ports in valves 61 and 62. Valves and 63 have a secondposition diverting flow in line 18 through line 65 to the conductor 12in which valve 63 is connected, closing flow from line 18 to manifold 19and closing flow from manifold 20 into conductor 12 in which valve 63 isconnected. Similarly, valves 61 and 62 have a second position divertingflow from conductor 12 in which valve 62 is located to line 21, closingflow from manifold 20 into line 21 and closing flow from manifold 20into conductor 12 in which valve 62 is located.

In operation, liquid nitrogen, LN in line 18 entering power circuit 10at an inlettemperature typically of 65 K. and the 20 atmospherespressure flows through the conductors 12 of circuits 10 and 11 and isreturned to line 21 on the suction side of circulator 24. Thecirculating coolant LN, encounters a pressure drop of 10 atmospheres incircuits 10 and 1 1 and rises to a temperature of 75 K. thusaccommodating not only the Joule heat losses in circuits 10 and 11 butalso those losses introduced by terminal heat load and fixed losses fromthe environment, e.g., the ambient soil to the coolant in conductor 12.Ideally, cryogenerator 26 has a refrigerating capacity capable ofmaintaining these inlet and outlet temperatures for some load oncircuits 10 and 11 intermediate cyclic peak loads and minimum loads withall of the neon refrigerant flowing from cryogenerator 26 through heatexchanger 27, Le, in the second mode of valve 56 and first of valve 58.In this state, it will be appreciated that at some earlier time theliquid nitrogen in reservoir 25 was subcooled to 65 K. with vent valve34 closed leaving a subatmospheric pressure in the space above theliquid level in tank 25.

Typically the neon entering coil 33 of tank 25 from valves 58 and 56 andline 46 is at 635 K. and 10.66 atmospheres and is heated in coil 33 to73.8 K. valve 5% is adjusted on variation of cooling requirements todivide the flow between line 57 and heat exchanger 27 to maintain thistemperature. In the cycle in cryogenerator 26, neon absorbs heat inelement 37 and enters compressor 39 at 298.5 K. and 10.54 atmospheres.ln compressor 39 the neon is compressed to 20 atmospheres and heats to300 K., additional heat being rejected from the circuit at this point.On returning through element 38 of heat exchanger 35 the neon is cooledto 77.9" K. and then is further cooled in expander 40 back to 63.5 K.for recycle to tank 25 and heat exchanger 27.

When the transmission line, including circuits and 11, is required tocarry a load in excess of its continuous rating, i.e., when the coolingrequirements exceed the refrigerating capacity of cryogenerator 26, thefirst indication is that the output temperature of liquid nitrogen inline 21 increases above 75 K. Since the output of cryogenerator 26 isconstant, the input temperature of liquid nitrogen to cable 10 in line18 will show a corresponding increase above 65 K.

At this point, valve 29 is adjusted to divide the circulating coolantliquid nitrogen, LN between line 30 and line 31. The change from thefirst mode of operation of valve 29 to the second is gradual so that abalance can be struck between flow in line 31 and the flow directly toline 30 from valve 29 such that the inlet temperature to circuit 10 inline 18 can be kept at 65 K. As the temperature of the liquid nitrogenin tank 25 increases, vent valve 34, which is a pressure relief valve,is opened at 77 K. (When the vapor pressure of liquid nitrogen equalsone atmosphere). Cold nitrogen vaporis passed in line 47 tocryogenerator 26, where in element 36 it is used to aid in extractingheat from the neon before venting the nitrogen vapor to atmosphere. Thiscondition of operation can be maintained indefinitely to a point inwhich the outlet temperature of liquid nitrogen leaving circuit 11approaches 103 K., and so long as the supply of liquid nitrogen in tank25 is replenished.

When the outlet temperature of liquid nitrogen in line 21 approximates103 K. (vaporization temperature at 10 atmospheres) and the inlettemperature of liquid nitrogen in line 18 is still at 65 K., maximumheat is removed from the double circuit system. These conditions definethe heat exchange requirements in tank 25 and heat exchanger 27 and thesize of reservoir required in tank 25. v

After a period of maximum sustained overload, the load in the systemreturns to normal full load rating, i.e., the load matched by thecryogenerator 26 refrigerating capacity, the outlet temperature ofliquid nitrogen in line 21 will fall, and the rate of vaporization ofliquid nitrogen in tank 25 will decrease until the outlet temperature ofliquid nitrogen in line 21 returns to 77 K. At this point, valve 29shunts all flow of coolant past tank 25 and vent valve 34 closes. Theoutlet temperature in line 21 will continue to decrease until it reaches75 K. at which steady state conditions return.

Sooner or later, the load will dip below full load level. Such periodsin which the cooling capacity required in circuits 10 and 11 is lessthan the capability of cryogenerator 26 must be used to subcool thestored liquid nitrogen in tank 25. At this point, i.e., when thetemperature of liquid nitrogen in line 21 falls below 75 K., valve 58 isoperated to divert some or all neon refrigerant from line 57 throughcoil 33 until the temperature in tank 25 decreases to about 75 K. Atthat point valve 56 is operated, as the loading conditions on cable 10and 11 permit, to pass some or all of the neon refrigerant in line 46from cryogenerator 26 directly to coil 33 in tank 25. This procedure mayhave to be repeated several times until the tank 25 temperature falls toabout 65 K. During periods of subcooling of the stored liquid nitrogenin tank 25, the inlet and outlet temperatures in lines 18 and 21 can beallowed to increase a few degrees depending upon anticipated loadconditions at the time. On completion of the subcooling process, andassuming no overload has to be supported, the inlet temperature at line18 is then reduced to 65 K. and the outlet temperature in line 21settles at some equilibrium less than or approximating 75 K. at whichpoint the system has recovered its full overload capability.

Referring to FIG. 1, it will be appreciated at anytime during operationof power circuits l0 and 11, should there occur a breakdown in one powercircuit requiring shutdown of the circuit for repair or otherwisedisabling the circuit, the entire load of the transmission line can bethrown on the other power circuit. Such will result in essentiallyquadrupling the Joule heat losses in the circuit, but by rerouting theflow of liquid nitrogen coolant LN, through the single circuit takingthe load, the cooling capacity of that circuit can also be increased toaccommodate the new load, at least temporarily. in such an arrangement,valve 64 is closed. Valves 60 and 63 are operated to divert flow fromline 18 through line 65 when circuit 11 is to remain in service, andvalves 61 and 62 are operated to divert flow through line 66 whencircuit 10 is to remain in service. In each case flow is serially in onedirection in one conductor 12 and in the reverse direction in the othertwo conductors 12 of the circuit 10 or 11 remaining in service.

Referring more particularly to H6. 5, provision is made foraccommodating the transmission line constituted by circuits 10 and 11 tolong term growth in power-handling requirements of the transmissionline. In accordance with this invention, when the load-handlingcapability of the transmission line shown in FIG. 1 has reached themaximum that can be handled without employing additional transmissionlines, the capacity of the transmission line can be increased withoutthe addition of new cables by adding refrigerating capacity in the formof additional cryogenerators or complete refrigerating plants 17' atintervals along the transmission line thus effectively increasing therefrigerating capacity of each segment of the transmission line andhence its power-handling capability. Thus in accordance with thisinvention refrigeration capacity of a given transmission line isincreased in stages to accommodate long term increase in load.Typically, the line is operated initially with one small refrigeratingplant at one station on the line. Next a larger plant is installed atthe same location. Then a second plant is added at a second station onthe line. Thereafter intermediate stations are added.

We claim:

1. A coolant circuit in combination with a resistive cryogenic electricpower transmission line, which power line includes electric circuitmeans having a conductor, which coolant circuit includes:

A. First heat exchange means for containing a coolant in heat exchangerelationship with said conductor;

B. A reservoir for containing a vaporizable liquid and including i.second heat exchange means for containing said coolant in heat exchangerelationship with said liquid;

C. Refrigeration means including i. third heat exchange means forcontaining said coolant to extract heat therefrom;

D. Means for circulating said coolant through said first,

second and third heat exchange means,

i. said circulating means being normally operable to circulate saidcoolant through said first and third heat exchange means when thecooling requirements imposed on the coolant circuit are met by or areexceeded by the refrigerating capacity of said refrigerating means, and

ii. said circulating means being operable to circulate said coolantthrough said first, second and third heat exchange means when saidcooling requirements exceed said refrigerating capacity; an;

E. Vent means connected to said reservoir,

i. said vent means being normally closed, and

ii. said vent means being operable upon a rise of the temperature ofsaid liquid contained in said reservoir to a preselected value to permitescape from said reservoir of vaporized liquid.

2. The combination according to claim 1 in which B. said refrigeratingmeans further includes ii. fourth heat exchange means located in heatexchange relationship with said liquid in said reservoir operable toextract heat from said liquid when said cooling requirements areexceeded by said refrigerating capacity.

3. The combination according to claim 1 in which said coolant is aliquefiable gas and which further includes F. Means for maintainingsuperatmospheric pressure on said coolant whereby said coolant ismaintained in the liquid state.

' 4. The combination according to claim 1 for a resistive cryogenicelectric power transmission line in which said power line includes apair of electrically parallel electrical power circuits, each electriccircuit having at least two conductors.

A. Said first heat exchange means having portions for containing saidcoolant in heat exchange relationship with each of said conductors; and

D. Said circulating means further including iii. connections wherebysaid coolant is normally circulated in a loop through said first heatexchange means with the portions thereof associated with the conductorsin each electric circuit connected in parallel and with the portionsthereof associated with the conductors in one electric circuit connectedin series with the portions thereof associated with the conductors ofthe other electric circuit, and

iv. connections whereby said coolant can be circulated in series throughthe portions of said first heat exchange means associated with theconductors of one said electric circuit to the exclusion of suchportions associated with the conductors of the other said electriccircuit.

5. A coolant circuit in combination with a resistive cryogenic electricpower transmission line in which said power line includes a pair ofelectrically parallel electrical power circuits, each electric circuithaving at least two conductors,

A. First heat exchange means having portions for containing said coolantin heat exchange relationship with each of said conductors;

B. Refrigeration means including i. second heat exchange means forcontaining said coolant to extract heat therefrom;

C. Means for circulating said coolant including i. connections wherebysaid coolant is normally circulated in a loop through said first andsecond heat exchange means with the portions of said first heat exchangemeans associated with the conductors in each electric circuit connectedin parallel and with the portions of said first heat exchange meansassociated with the conductors in one electric circuit connected inseries with the portions thereof associated with the conductors of theother electric circuit, and

ii. connections whereby said coolant can be circulated in series throughthe portions of said first heat exchange means associated with theconductors of one said electric circuit and said second heat exchangemeans to the exclusion of such portions associated with the conductorsof the other said electric circuit.

6. A method for cooling a resistive cryogenic electric powertransmission line which includes circulating a coolant in heat exchangerelationship with the elements of a power line to be cooled, maintaininga refrigeration zone for extracting heat from said coolant, maintaininga body of vaporizable liquid, circulating said coolant between heatexchange relationship with said transmission line elements and heatexchange relationship with said refrigeration zone during periods inwhich the cooling requirements of said elements are met by or areexceeded by the refrigerating capacity of said refrigeration zone,circulating said coolant in heat exchange relationship with said body ofliquid when said cooling requirements exceed said refrigeratingcapacity, and venting vaporized liquid from said body of liquid when thetemperature thereof rises above a preselected value.

7. A method according to claim 6 which additionally includes utilizingthe refrigerating capacity of said refrigeration zone to extract heatfrom said body of liquid during periods in which said coolingrequirements are exceeded by said refrigerating capacity.

8. A method according to claim 6 in which said coolant is a liquefiablegas and which further includes maintaining superatmospheric pressure onsaid coolant whereby said coolant is maintained in liquid state.

9. A method according to claim 8 which further includes utilizing therefrigerating capacity of said refrigeration zone to lower thetemperature of said coolant substantially below its vaporizationtemperature under the maintained superatmospheric pressure duringperiods in which said cooling requirements are exceeded by saidrefrigerating capacity.

10. A method for cooling a resistive cryogenic electric powertransmission line which includes circulating a coolant in heat exchangerelationship with the elements of a power line to be cooled, saidcoolant being a liquefiable gas, maintaining superatmosphen'c pressureon said coolant whereby said coolant is maintained in liquid state,maintaining a refrigeration zone for extracting heat from said coolant,circulating said coolant between heat exchange relationship with saidtransmission line elements and heat exchange relationship with saidrefrigeration zone, and utilizing the refrigerating capacity of saidrefrigeration zone to lower the temperature of said coolantsubstantially below its vaporization temperature under the maintainedsuperatmospheric pressure during periods in which said coolingrequirements are exceeded by said refrigerating capacity.

1. A coolant circuit in combination with a resistive cryogenic electricpower transmission line, which power line includes electric circuitmeans having a conductor, which coolant circuit includes: A. First heatexchange means for containing a coolant in heat exchange relationshipwith said conductor; B. A reservoir for containing a vaporizable liquidand including i. second heat exchange means for containing said coolantin heat exchange relationship with said liquid; C. Refrigeration meansincluding i. third heat exchange means for containing said coolant toextract heat therefrom; D. Means for circulating said coolant throughsaid first, second and third heat exchange means, i. said circulatingmeans being normally operable to circulate said coolant through saidfirst and third heat exchange means when the cooling requirementsimposed on the coolant circuit are met by or are exceeded by therefrigerating capacity of said refrigerating means, and ii. saidcirculating means being operable to circulate said coolant through saidfirst, second and third heat exchange means when said coolingrequirements exceed said refrigerating capacity; and E. Vent meansconnected to said reservoir, i. said vent means being normally closed,and ii. said vent means being operable upon a rise of the temperature ofsaid liquid contained in said reservoir to a preselected value to permitescape from said reservoir of vaporized liquid.
 2. The combinationaccording to claim 1 in which B. said refrigerating means furtherincludes ii. fourth heat exchange means loCated in heat exchangerelationship with said liquid in said reservoir operable to extract heatfrom said liquid when said cooling requirements are exceeded by saidrefrigerating capacity.
 3. The combination according to claim 1 in whichsaid coolant is a liquefiable gas and which further includes F. Meansfor maintaining superatmospheric pressure on said coolant whereby saidcoolant is maintained in the liquid state.
 4. The combination accordingto claim 1 for a resistive cryogenic electric power transmission line inwhich said power line includes a pair of electrically parallelelectrical power circuits, each electric circuit having at least twoconductors. A. Said first heat exchange means having portions forcontaining said coolant in heat exchange relationship with each of saidconductors; and D. Said circulating means further including iii.connections whereby said coolant is normally circulated in a loopthrough said first heat exchange means with the portions thereofassociated with the conductors in each electric circuit connected inparallel and with the portions thereof associated with the conductors inone electric circuit connected in series with the portions thereofassociated with the conductors of the other electric circuit, and iv.connections whereby said coolant can be circulated in series through theportions of said first heat exchange means associated with theconductors of one said electric circuit to the exclusion of suchportions associated with the conductors of the other said electriccircuit.
 5. A coolant circuit in combination with a resistive cryogenicelectric power transmission line in which said power line includes apair of electrically parallel electrical power circuits, each electriccircuit having at least two conductors, A. First heat exchange meanshaving portions for containing said coolant in heat exchangerelationship with each of said conductors; B. Refrigeration meansincluding i. second heat exchange means for containing said coolant toextract heat therefrom; C. Means for circulating said coolant includingi. connections whereby said coolant is normally circulated in a loopthrough said first and second heat exchange means with the portions ofsaid first heat exchange means associated with the conductors in eachelectric circuit connected in parallel and with the portions of saidfirst heat exchange means associated with the conductors in one electriccircuit connected in series with the portions thereof associated withthe conductors of the other electric circuit, and ii. connectionswhereby said coolant can be circulated in series through the portions ofsaid first heat exchange means associated with the conductors of onesaid electric circuit and said second heat exchange means to theexclusion of such portions associated with the conductors of the othersaid electric circuit.
 6. A method for cooling a resistive cryogenicelectric power transmission line which includes circulating a coolant inheat exchange relationship with the elements of a power line to becooled, maintaining a refrigeration zone for extracting heat from saidcoolant, maintaining a body of vaporizable liquid, circulating saidcoolant between heat exchange relationship with said transmission lineelements and heat exchange relationship with said refrigeration zoneduring periods in which the cooling requirements of said elements aremet by or are exceeded by the refrigerating capacity of saidrefrigeration zone, circulating said coolant in heat exchangerelationship with said body of liquid when said cooling requirementsexceed said refrigerating capacity, and venting vaporized liquid fromsaid body of liquid when the temperature thereof rises above apreselected value.
 7. A method according to claim 6 which additionallyincludes utilizing the refrigerating capacity of said refrigeration zoneto extract heat from said body of liquid during periods in which saidcooling requiremEnts are exceeded by said refrigerating capacity.
 8. Amethod according to claim 6 in which said coolant is a liquefiable gasand which further includes maintaining superatmospheric pressure on saidcoolant whereby said coolant is maintained in liquid state.
 9. A methodaccording to claim 8 which further includes utilizing the refrigeratingcapacity of said refrigeration zone to lower the temperature of saidcoolant substantially below its vaporization temperature under themaintained superatmospheric pressure during periods in which saidcooling requirements are exceeded by said refrigerating capacity.
 10. Amethod for cooling a resistive cryogenic electric power transmissionline which includes circulating a coolant in heat exchange relationshipwith the elements of a power line to be cooled, said coolant being aliquefiable gas, maintaining superatmospheric pressure on said coolantwhereby said coolant is maintained in liquid state, maintaining arefrigeration zone for extracting heat from said coolant, circulatingsaid coolant between heat exchange relationship with said transmissionline elements and heat exchange relationship with said refrigerationzone, and utilizing the refrigerating capacity of said refrigerationzone to lower the temperature of said coolant substantially below itsvaporization temperature under the maintained superatmospheric pressureduring periods in which said cooling requirements are exceeded by saidrefrigerating capacity.